Micropump

ABSTRACT

The present invention generally provides a micropump that utilizes electroosmotic pumping of fluid in one channel or region to generate a pressure based flow of material in a connected channel, where the connected channel has substantially no electroosmotic flow generated. Such pumps have a variety of applications, and are particularly useful in those situations where the application for which the pump is to be used prohibits the application of electric fields to the channel in which fluid flow is desired, or where pressure based flow is particularly desirable.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/709,739, filed Nov. 9, 2000 now U.S. Pat. No. 6,394,759, which is acontinuation of U.S. patent application Ser. No. 09/420,987, filed Oct.20, 1999, now U.S. Pat. No. 6,171,067, which is a continuation of U.S.patent application Ser. No. 08/937,958, filed Sep. 25, 1997, now U.S.Pat. No. 6,012,902.

BACKGROUND OF THE INVENTION

The field of microfluidics has gained substantial attention as apotential answer to many of the problems inherent in conventionalchemical, biochemical and biological analysis, synthesis andexperimentation. In particular, by miniaturizing substantial portions oflaboratory experimentation previously performed at a lab bench, one cangain substantial advantages in terms of speed, cost, automatability, andreproducibility of that experimentation. This substantial level ofattention has led to a variety of developments aimed at accomplishingthat miniaturization, e.g., in fluid and material handling, detectionand the like.

U.S. Pat. No. 5,271,724 to van Lintel, for example reports a microscalepump/valve assembly fabricated from silicon using manufacturingtechniques typically employed in the electronics and semiconductorindustries. The microscale pump includes a miniature flexible diaphragmas one wall of a pump chamber, and having a piezoelectric elementmounted upon its exterior surface.

Similarly, U.S. Pat. No. 5,375,979 to Trah, reports a mechanicalmicropump/valve assembly that is fabricated from three substrate layers.The pump/valve assembly consists of a top cover layer disposed over amiddle layer having a cavity fabricated therein, to define the pumpingchamber. The bottom layer is mated with the middle layer and together,these substrates define each of two, one way flap valves. The inletvalve consists of a thin flap of the middle substrate layer that isdisposed over an inlet port in the bottom substrate layer, and seatedagainst the bottom layer, such that the flap valve will only open inwardtoward the pump chamber. A similar but opposite construction is used onthe outlet valve, where the thin flap is fabricated from the bottomlayer, is seated over the outlet port and against the middle layer suchthat the valve only opens away from the pump chamber. The pump andvalves cooperate to ensure that fluid moves in only one direction.

Published PCT Application No. 97/02357 reports an integratedmicrofluidic device incorporating a microfluidic flow system incombination with an oligonucleotide array. The microfluidic system movesfluid by application of external pressures, e.g., via a pneumaticmanifold, or through the use of diaphragm pumps and valves.

While these microfabricated pumps and valves provide one means oftransporting fluids within microfabricated substrates, their fabricationmethods and materials can be somewhat complex, resulting in excessivevolume requirements, as well as resulting in an expensive manufacturingprocess.

Published PCT Application No. 96/04547 to Ramsey, describes an elegantmethod of transporting and directing fluids through an interconnectedchannel structure using controlled electrokinetic forces at theintersections of the channels, to control the flow of material at thoseintersections. These material transport systems employ electrodesdisposed in contact with the various channel structures to apply thecontrolled electrokinetic forces. These methods have been adapted for avariety of applications, e.g., performing standard assays, screening oftest compounds, and separation/sequencing of nucleic acids, and thelike. See, e.g., commonly assigned U. S. Pat. No. 6,046,056, U. S.patent application Ser. No. 60/086,240, filed Apr. 4, 1997 and U. S.Pat. No. 5,976,336, all of which are incorporated herein by reference inits entirety for all purposes. These “solid state” material transportsystems combine a high degree of controllability with an ease ofmanufacturing.

Despite the numerous advantages of using controlled electrokineticmaterial transport in microfluidic systems, in some cases it isdesirable to combine the ease of control and fabrication attendant tosuch systems with the benefits of pressure-based fluid transportsystems. The present invention meets these and other needs.

SUMMARY OF THE INVENTION

The present invention provides microfluidic systems that incorporate theease of fabrication and operation of controlled electrokinetic materialtransport systems, with the benefits of pressure-based fluid flow inmicrofluidic systems. The present invention accomplishes this byproviding, in a first aspect, a microfluidic device having a bodystructure with at least one microscale channel disposed therein, andalso having an integrated micropump in fluid communication with themicroscale channel. The micropump comprises a first microscale channelportion having first and second ends, and a second microscale channelportion having first and second ends. The second channel portion has afirst effective surface charge associated with its walls. The first endof the second channel portion is in fluid communication with the firstend of the first channel portion at a first channel junction. The pumpalso includes a means for applying a voltage gradient between the firstand second ends of the second channel portion while applyingsubstantially no voltage gradient between the first and second ends ofthe first channel portion.

The microfluidic devices and micropumps of the present invention mayalso include a third channel portion that is in communication with thechannel junction, and which includes a charge associated with itssurface. This charge may be the same as or substantially opposite tothat of the second channel portion. This third channel portion alsotypically includes a means for applying a voltage gradient across itslength, which means may be the same as or different from that used toapply a voltage gradient across the length of the second channelportion.

In a related aspect, the present invention also provides a method oftransporting fluid in a microfluidic channel structure, which comprisesproviding a micropump of the present invention. The method alsocomprises applying an appropriate voltage gradient along the length ofthe second channel portion to produce an electroosmotically inducedpressure within the second channel portion. This is followed by thetransmission of that pressure to the first channel portion whereuponpressure-based flow is achieved in that first channel.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic illustration of one embodiment of a microscaleelectroosmotic pressure pump according to the present invention.

FIG. 2 illustrates an alternate embodiment of a pressure pump accordingto the present invention, incorporating a flow restrictive channel forshunting of the current used to drive electroosmotic flow.

FIG. 3 illustrates still another embodiment of a micropump according tothe present invention. As shown the micropump includes two pumpingchannels having oppositely charged surfaces.

FIG. 4 is a schematic illustration of a microfluidic device for carryingout continuous enzyme/inhibitor screening assays, and incorporatingseveral integrated micropumps according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally provides a micropump that utilizeselectroosmotic pumping of fluid in one channel or region to generate apressure based flow of material in a connected channel, where theconnected channel has substantially no electroosmotic flow generated.Such pumps have a variety of applications, and are particularly usefulin those situations where the application for which the pump is to beused prohibits the application of electric fields to the channel inwhich fluid flow is desired, or where pressure based flow isparticularly desirable. Such applications include those involving thetransport of materials that are not easily or predictably transported byelectrokinetic flow systems, e.g.: materials having high ionicstrengths; non-aqueous materials; materials having electrophoreticmobilities that detract from bulk electroosmotic material transport; ormaterials which interact with the relevant surfaces of the system,adversely affecting electrokinetic material transport.

Alternatively, in some instances pressure based flow is desirable forother reasons. For example, where one wishes to expel materials from theinterior portion or channels of a microfluidic system, or to deliver amaterial to an external analytical system, it may be impracticable toelectrokinetically transport such materials over the entire extent ofthe ultimate flow path. Examples of the above instances includeadministration of pharmaceutical compounds for human or veterinarytherapy, or for administration of insecticides, e.g., in veterinaryapplications.

The micropumps of the present invention typically utilize and are madeup of channels incorporated into microfluidic device or system in whichsuch pumps are to be used. By “microfluidic device or system” istypically meant a device that incorporates one or more interconnectedmicroscale channels for conveying fluids or other materials. Typically,the microscale channels are incorporated within a body structure. Thebody structure of the microfluidic devices described herein typicallycomprises an aggregation of two or more separate layers which whenappropriately mated or joined together, form the microfluidic device ofthe invention, e.g., containing the channels and/or chambers describedherein. Typically, the microfluidic devices described herein willcomprise a top portion, a bottom portion, and an interior portion,wherein the interior portion substantially defines the channels andchambers of the device.

As used herein, the term microscale refers to channel structures whichhave at least one cross-sectional dimension, i.e., width, depth ordiameter, that is between about 0.1 and 500 μm, and preferably, betweenabout 1 and about 200 μm. In particularly preferred aspects, a channelfor normal material transport will be from about 1 to about 50 μm deep,while being from about 20 to about 100 μm wide. These dimensions mayvary in cases where a particular application requires wider, deeper ornarrower channel dimensions, e.g., as described below.

In preferred aspects, the microfluidic devices incorporating themicropumps according to the present invention utilize a two-layer bodystructure. The bottom portion of the device typically comprises a solidsubstrate which is substantially planar in structure, and which has atleast one substantially flat upper surface. A variety of substratematerials may be employed as the bottom portion. Typically, because thedevices are microfabricated, substrate materials will be selected basedupon their compatibility with known microfabrication techniques, e.g.,photolithography, wet chemical etching, laser ablation, air abrasiontechniques, injection molding, embossing, and other techniques. Thesubstrate materials are also generally selected for their compatibilitywith the full range of conditions to which the microfluidic devices maybe exposed, including extremes of pH, temperature, salt concentration,and application of electric fields. Accordingly, in some preferredaspects, the substrate material may include materials normallyassociated with the semiconductor industry in which suchmicrofabrication techniques are regularly employed, including, e.g.,silica based substrates, such as glass, quartz, silicon or polysilicon,as well as other substrate materials, such as gallium arsenide and thelike. In the case of semiconductive materials, it will often bedesirable to provide an insulating coating or layer, e.g., siliconoxide, over the substrate material, and particularly in thoseapplications where electric fields are to be applied to the device orits contents.

In additional preferred aspects, the substrate materials will comprisepolymeric materials, e.g., plastics, such as polymethylmethacrylate(PMMA), polycarbonate, polytetrafluoroethylene (TEFLON™),polyvinylchloride (PVC), polydimethylsiloxane (PDMS), polysulfone, andthe like. Such polymeric substrates are readily manufactured usingavailable microfabrication techniques, as described above, or frommicrofabricated masters, using well known molding techniques, such asinjection molding, embossing or stamping, or by polymerizing thepolymeric precursor material within the mold (See U.S. Pat. No.5,512,131). Such polymeric substrate materials are preferred for theirease of manufacture, low cost and disposability, as well as theirgeneral inertness to most extreme reaction conditions.

As described in greater detail below, the channel portions of thedevices of the present invention typically include, at least in part,channel surfaces that have charged functional groups associatedtherewith, in order to produce sufficient electroosmotic flow togenerate the requisite pressures in those channels in which noelectroosmotic flow is taking place. In the case of silica basedsubstrates, negatively charged hydroxyl groups present upon the etchedsurfaces of the channels are typically more than sufficient to generatesufficient electroosmotic flow upon application of a voltage gradientalong such channels. In the case of other substrate materials, or caseswhere substantially no surface charge, or a positive surface charge isrequired, the surface of these channels is optionally treated to providesuch surface charge. A variety of methods may be used to providesubstrate materials having an appropriate surface charge, e.g.,silanization, application of surface coatings, etc. Use of such surfacetreatments to enhance the utility of the microfluidic system, e.g.,provide enhanced fluid direction, is described in U.S. Pat. No.5,885,470, which is incorporated herein by reference in its entirety forall purposes.

The channels and/or chambers of the microfluidic devices are typicallyfabricated into the upper surface of the bottom substrate or portion, asmicroscale grooves or indentations, using the above describedmicrofabrication techniques. The top portion or substrate also comprisesa first planar surface, and a second surface opposite the first planarsurface. In the microfluidic devices prepared in accordance with themethods described herein, the top portion also includes a plurality ofapertures, holes or ports, disposed therethrough, e.g., from the firstplanar surface to the second surface opposite the first planar surface.

The first planar surface of the top substrate is then mated, e.g.,placed into contact with, and bonded to the planar surface of the bottomsubstrate, covering and sealing the grooves and/or indentations in thesurface of the bottom substrate, to form the channels and/or chambers(i.e., the interior portion) of the device at the interface of these twocomponents. The holes in the top portion of the device are oriented suchthat they are in communication with at least one of the channels and/orchambers formed in the interior portion of the device from the groovesor indentations in the bottom substrate. In the completed device, theseholes function as reservoirs for facilitating fluid or materialintroduction into the channels or chambers of the interior portion ofthe device, as well as providing ports at which electrodes may be placedinto contact with fluids within the device, allowing application ofelectric fields along the channels of the device to control and directfluid transport within the device. Although the terms “port” and“reservoir” are typically used to describe the same general structuralelement, it will be readily appreciated that the term “port” generallyrefers to a point at which an electrode is placed into electricalcontact with the contents of a microfluidic channel or system.Similarly, the term “reservoir” typically denotes a chamber or wellwhich is capable of retaining fluid that is to be introduced into thevarious channels or chambers of the device. Such reservoirs may or maynot have an associated electrode, i.e., functioning as a port.

In many embodiments, the microfluidic devices will include an opticaldetection window disposed across one or more channels and/or chambers ofthe device. Optical detection windows are typically transparent suchthat they are capable of transmitting an optical signal from thechannel/chamber over which they are disposed. Optical detection windowsmay merely be a region of a transparent cover layer, e.g., where thecover layer is glass or quartz, or a transparent polymer material, e.g.,PMMA, polycarbonate, etc. Alternatively, where opaque substrates areused in manufacturing the devices, transparent detection windowsfabricated from the above materials may be separately manufactured intothe device.

These devices may be used in a variety of applications, including, e.g.,the performance of high throughput screening assays in drug discovery,immunoassays, diagnostics, genetic analysis, and the like. As such, thedevices described herein, will often include multiple sampleintroduction ports or reservoirs, for the parallel or serialintroduction and analysis of multiple samples. Alternatively, thesedevices may be coupled to a sample introduction port, e.g., a pipettor,which serially introduces multiple samples into the device for analysis.Examples of such sample introduction systems are described in e.g., U.S.Pat. Nos. 6,046,056 and 5,880,071, and is hereby incorporated byreference in its entirety for all purposes.

As noted, the micropumps described herein typically comprise, at leastin part, the microscale channels that are incorporated into the overallmicrofluidic device. In particular, such pumps typically include a firstmicroscale channel portion having first and second ends that is in fluidcommunication with a second channel portion at a first channel junction.The second channel portion typically has a surface charge associatedwith the walls of that channel portion, which charge is sufficient topropagate adequate levels of electroosmotic flow, specifically, the flowof fluid and material within a channel or chamber structure whichresults from the application of an electric field across suchstructures.

In brief, when a fluid is placed into a channel which has a surface,bearing charged functional groups, e.g., hydroxyl groups in etched glasschannels or glass microcapillaries, those groups can ionize. The natureof the charged functional groups can vary depending upon the nature ofthe substrate and the treatments to which that substrate is subjected,as described in greater detail, below. In the case of hydroxylfunctional groups, this ionization, e.g., at neutral pH, results in therelease of protons from the surface into the fluid, resulting in alocalization of cationic species within the fluid near the surface, or apositively charged sheath surrounding the bulk fluid in the channel.Application of a voltage gradient across the length of the channel, willcause the cation sheath to move in the direction of the voltage drop,i.e., toward the negative electrode, moving the bulk fluid along withit.

As noted above, the channel portions are typically fabricated into aplanar solid substrate. A voltage gradient is applied across the lengthof the second channel portion via electrodes disposed in electricalcontact with those ends, whereupon the voltage gradient causeselectroosmotic flow of fluid within the second channel portion. Thepressure developed from this electroosmotic flow is translated throughthe channel junction to the first channel portion. In accordance withthe present invention, the first channel portion produces substantiallyno electroosmotic flow, by virtue of either or both of: (1)a lack ofcharged groups on the surfaces or walls of the first channel; or (2) theabsence of a voltage gradient applied across the length of the firstchannel. As a result, the sole basis for material flow within the firstchannel portion is a result of the translation of pressure from thesecond channel portion to the first.

FIG. 1 illustrates a simplified schematic illustration of a micropump100 according to the present invention. As shown, the pump includes amicroscale channel structure 102 which includes a first channel portion104 and a second channel portion 106 that are in fluid communication ata channel junction point 108. Second channel portion 106 is shown asincluding charged functional groups 110 on its wall surfaces. Althoughillustrated as negatively charged groups, it will be appreciated thatpositively charged functional groups are optionally present on thesurface of the channels. The direction of fluid flow depends upon thedirection of the voltage gradient applied as well as the nature of thesurface charge, e.g., substantially negative or substantially positive.By “substantially negative” or “substantially positive” is meant that ina given area of the channel surface, the surface charge is net negativeor net positive. As such, some level of mixed charge is tolerated,provided it does not detract significantly from the application of thechannel, e.g., in propagating sufficient electroosmotic flow, e.g.,whereby those surfaces or channel walls are capable of supporting anelectroosmotic mobility (μEO) of at least about 1×10⁻⁵cm²V⁻¹s⁻¹, for astandard sodium borate buffer having an ionic strength of between about1 mM and about 10 mM, at a pH of from about 7 to about 10, disposedwithin those channels.

Differential surface charges, whether oppositely charged, or havingvaried charge densities among two or more channels, may be achieved bywell known methods. For example, surfaces are optionally treated withappropriate coatings, e.g., neutral or charged coatings, chargeneutralizing or charge adding reagents, e.g., protecting or cappinggroups, silanization reagents, and the like, to enhance chargedensities, and/or to provide net opposite surface charges, e.g., usingaminopropylsilanes, hydroxypropylsilanes, and the like.

Electrodes 112 and 114 are shown disposed in electrical contact with theends of the second channel portion. These electrodes are in turn,coupled to power source 116, which delivers appropriate voltages to theelectrodes to produce the requisite voltage gradient. Application of avoltage gradient between electrode 112 and electrode 114, e.g., a highervoltage applied at electrode 112, results in the propagation ofelectroosmotic flow within the second channel portion 106, asillustrated by arrow 118, while producing substantially noelectroosmotic flow in the first channel portion. Electroosmotic flow isavoided in the first channel portion by either providing the firstchannel portion with substantially no net surface charge to propagateelectroosmotic flow, or alternatively and preferably, electroosmoticflow is avoided in the first channel portion by applying substantiallyno voltage gradient across the length of this channel portion. Thephrase “applying substantially no voltage gradient across the firstchannel portion,” means that no electrical forces are applied to theends of the first channel portion whereby a voltage gradient isgenerated therebetween.

The electroosmotic flow of material in the second channel portion 106,produces a resultant pressure which is translated through channeljunction 108 to the first channel portion 104, resulting in a pressurebased flow of material in the first channel portion 104, as shown byarrow 120.

In particularly preferred aspects, the channel portion responsible forpropagating electroosmotic fluid flow, e.g., the second channel portion106, will include a narrower cross-sectional dimension, or will includea portion that has a narrower cross-sectional dimension than theremainder of the microscale channels in the overall channel structure,i.e., the first channel portion. In particular, electrokinetic flowvelocity of material in a microscale channel or capillary is independentof the diameter of the channel or capillary in which such flow is takingplace. As such, the flow volume is directly proportional to the crosssectional area of the channel. For a rectangular channel of width (“w”)and height (“h”) where h<<w, the flow volume is proportional to h for agiven w. In contrast, however, for poiseulle flow, the how volume for agiven pressure is inversely proportional to h³. It follows therefore,that as the height of the capillary channel is decreased, greater andgreater pressures are required to counteract the prevailingelectroosmotic flow. Accordingly, by reducing the height of a channel inwhich fluids are being pumped electroosmotically, one can significantlyincrease the amount of pressure produced thereby (e.g., by a factor ofh²).

The precise dimensions of the channels used for propagating theincreased pressures, also termed “pumping channels,” typically variesdepending upon the particular application for which such pumping isdesired, e.g., the pressure needs of the application. Further, pressurelevels also increase with the length of the channel through which thematerial is being transported. Typically, these pumping channels will beanywhere in the microscale range. Generally, although not required, thepumping channels will be narrower or shallower than the non-pumpingchannels contained within the microfluidic device. Typically, althoughby no means always, such pumping channels will vary from the remaining,non-pumping channels of the device in only one of the width or depthdimensions. As such, these pumping channels will typically be less than75% as deep or wide as the remaining channels, preferably, less than 50%as deep or wide, and often, less than 25% and even as low as 10% or lessdeep or wide than the remaining channels of the device.

Although FIG. 1 schematically illustrates the point of electricalcontact between electrode 114 and channel junction 108, e.g., the port,as being disposed within the overall channel comprised of the first andsecond channel portions 104 and 106, respectively, in preferred aspects,it is desirable to avoid the placement of electrodes within microscalechannels. In particular, electrolysis of materials at the electrodewithin these channels can result in substantial gas production. Such gasproduction can adversely effect material transport in these channels,e.g., resulting in ‘vapor lock’, or substantially increasing the levelof resistance through a given channel.

As such, the electrodes are typically disposed in electricalcommunication with ports or reservoirs that are, in turn, in fluid andelectrical communication with relevant the channel portion. An exampleof this modified micropump structure is illustrated in FIG. 2.

As shown, the micropump 200 again includes channel structure 102, whichcomprises first channel portion 104 and second channel portion 106, influid communication at a channel junction 108. Again, the second channelportion includes walls having an appropriate surface charge 110, and aregion of narrowed cross-sectional dimension 206, to optimize the ratioof pressure to electroosmotic flow. Electrodes 112 and 114, are coupledto power source 116, and are in electrical contact with the ends ofsecond channel 106 via reservoirs 218 and 216, respectively. Again,these electrodes deliver an appropriate voltage gradient across thelength of the second channel portion 106.

In order to apply an appropriate voltage gradient across second channelportion 106 without placing electrode 114 into the channel through whichfluid movement is desired, i.e., at channel junction 108, the electrodeis instead placed in electrical communication with a side channel 202.As described for electrode placement above, this electrode is typicallydisposed within a reservoir 216 that is located at the unintersectedterminus of side channel 202. Side channel 202 typically includes anappropriate flow restrictive element 204. The flow restrictive elementis provided to allow passage of current between the two electrodes,while substantially preventing fluid flow through side channel 202, alsotermed a flow restrictive channel. As a result, the electroosmotic flowof fluid through second channel portion 106 translates it's associatedpressure into first channel portion 104.

In at least a first aspect, the flow restrictive element includes afluid barrier that prevents flow of fluid, but permits transmission ofelectrons or ion species, e.g., a salt bridge. Examples of suchmaterials include, e.g., agarose or polyacrylamide gel plugs disposedwithin the side channel 202. Alternatively, the side channel 202 maycomprise a series of parallel channels each having a much smallercross-sectional area than the remainder of the channel structure, toreduce electroosmotic flow through the side channel. Again, the width ordepth of these flow restrictive channels will depend upon theapplication for which the pump is to be used, i.e., depending upon theamount of pressure which they must withstand, provided again that theyare narrower or shallower than the remaining channels of the overalldevice. Typically, however, these small diameter channels will have atleast one cross sectional dimension in the range of from about 0.001 toabout 0.05 μm. Typically, this narrow cross-section will be the depthdimension, while the width of these channels be on the order of fromabout 0.1 to about 50 μm, and preferably, from about 1 to about 10 μm.This is as compared to the width of second channel portion whichtypically ranges from about 20 to about 100 μm. Side channel 202, whichoptionally includes a plurality of parallel channels, also substantiallylacks surface charge, to reduce or eliminate any electroosmotic flowalong the side channel 202.

FIG. 3 illustrates still another embodiment of the electroosmoticpressure pump according to the present invention. This embodiment of themicropump has the added advantage of not requiring a side channel toshunt off current, e.g., as shown in FIG. 2. In particular, as shown,the pump 300 includes a channel structure which is comprised of a firstchannel portion 104, a second channel portion 106, and a third channelportion 304, all of which are in fluid communication at the channeljunction 306. The second and third channel portions 106 and 304, includesubstantially different surface charges 110 and 308, respectively, ontheir surfaces or channel walls (shown as negative charged groups insecond channel portion 106 and positive charged groups in third channelportion 304). By “substantially different surface charge” is meant thattwo surfaces will have respective surface charges that are substantiallydifferent in charge density or substantially different in type ofcharge, e.g., positive versus negative. Substantially different chargedensities include two surfaces where one surface has a charge densitythat is at least 10% lower than the other surface, typically greaterthan 20% less, preferably, greater than 30% less, and more preferably,greater than 50% less. Determination of relative surface charge densityis typically carried out by known methods. For example, appropriatecomparisons are made by determination of surface potential as measuredby the surfaces' ability to propagate electroosmotic flow of a standardbuffer, as noted above. This also includes instances where one surfaceis neutral as compared to the other surface that bears a charge, eitherpositive or negative.

By “substantially oppositely charged,” is meant that the net charge ontwo surfaces are substantially opposite to each other, e.g., one issubstantially positive, while the other is substantially negative. Thus,each surface can have surface charges of each sign, provided that theoverall net charge of the surface is either positive, or negative.

The effect of these different surface charges in the second and thirdchannel portions, 106 and 304 respectively, is to propagate differentlevels of electroosmotic flow in these channels, e.g., either differentlevels of flow in the same direction, or flow in opposite directions.This different flow results in a creation of net pressure in the firstchannel portion 104. In the case of oppositely charged second and thirdchannel portions, as shown in FIG. 3, the effect is to propagateelectroosmotic flow in opposite directions, under the same voltagegradient. Electrodes 112 and 114 are then placed into electrical contactwith the second and third channel portions 106 and 304, at the ends ofthese channels opposite from the channel junction 306, e.g., atreservoirs 316 and 318, respectively. Application of a voltage gradientfrom electrode 112 to electrode 114 (high to low) results in anelectroosmotic flow of fluid within each of the second and third channelportions 106 and 304 toward the channel junction, as shown by arrows 310and 312. The convergence of the fluid flow from each of the second andthird channel portions 106 and 304 results in a pressure based flowwithin first channel portion 104, as shown by arrow 314. Again, each ofsecond and third channel portions is optionally provided with a narrowedcross-sectional dimension, at least as to a portion of the channelportion (not shown), relative to the remainder of the channel structure,so as to optimize the level of pressure produced by the pump. It isnotable that in the case of the micropump where the second channelportion is charged and the third channel portion is neutral, the pump isvirtually the same structure as that illustrated in FIG. 2, wherein theflow restrictive channel merely lacks a surface charge, instead ofincorporating a fluid barrier.

In addition to creating positive pressures in the first channel portion,it should be noted that by reversing the direction of the voltagegradient applied across the pumping channels, the flow and thus thepressure produced in the first channel portion will be reversed, e.g.,creating a negative pressure within the first channel portion. Suchdrawing pumps have a variety of uses including use as sampling systemsfor drawing samples into microfluidic analyzers, e.g., from sample wellsin microtiter plates, patients, and the like.

As noted above the pressure based micropumps of the present inventionhave a variety of uses. In particular, such micropumps combine the easeof fabrication and operation of electrokinetic material transportsystems, with the benefits attendant to pressure-based flow, such aslack of electrophoretic biasing, bulk flow of materials that areotherwise difficult to transport, e.g., under E/O flow, such as largeparticulate matter, etc.

In one preferred aspect, the pressure-based micropumps according to thepresent invention are useful as integrated fluid transport and directionsystems in microfluidic systems, which may in turn be used to performany of a variety of chemical, biochemical, biological or otheranalytical or synthetic operations as described above. In particular,these electroosmotic pressure pumps are readily incorporated into any ofa number of previously described microfluidic systems, e.g., thoseemploying purely mechanical fluid direction systems, or those employingpurely electrokinetic fluid direction systems. In the latter case, amicropump as described herein is readily substituted for each of theports in a controlled electrokinetic system. Controlled electrokineticsystems are described in detail in Published International ApplicationWO 96/04547, to Ramsey, which is incorporated herein by reference in itsentirety.

In alternate preferred aspects, the pressure-based micropumps of thepresent invention are useful for interfacing microfluidic devices withmore conventional systems, e.g., conventional analytical equipment, suchas mass spectrometers, HPLC, GC, etc. Specifically, these micropumps arecapable of injecting small amounts of fluid from a microfluidic systeminto a fluid interface to such equipment without requiring a potentialgradient through that interface.

Additionally, such micropumps are particularly useful for dispensingsmall amounts of fluid in a controlled manner, from a microfluidicsystem, device or storage vessel. For example, in preferred aspects,these pumps are useful in the controlled administration ofpharmaceutical compounds, e.g., in human or veterinary applications.Such devices may be placed against the skin of a patient, e.g., fortransdermal delivery, or alternatively, may be implanted subcutaneously,for direct administration. In an alternate example, such pumps areuseful in dispensing very small amounts of material for subsequentreaction or location, e.g., in combinatorial synthesis of chemicalspecies on substrate surfaces, i.e., high density chemical or polymerarrays.

EXAMPLES

As noted above, the micropumps of the present invention are readilyintegrated into a variety of microfluidic systems, including screeningassay systems, e.g., as described in commonly assigned U.S. Pat. NoS.6,046,056 and 5,964,995, and incorporated herein by reference in theirentirety.

FIG. 4 illustrates a continuous flow assay system used to perform enzymeinhibitor assays. The channel geometry of the device was previouslyutilized for this same purpose, but in conjunction with a controlledelectrokinetic transport system. As shown however, the individual portsof the electrokinetic device are each substituted with an electroosmoticpressure-based micropump according to the present invention.Specifically, an electroosmotic pressure pump including two separateport/reservoirs is placed at the originating end of the channels of thedevice. Together, each group of two port/reservoirs is termed a “pumpmodule.”

As shown, the device 400 is fabricated in a body structure 402 andincludes a main analysis channel 404, in which the enzyme/inhibitorscreening assays are carried out. A chromogenic, fluorogenic,chemiluminescent or fluorescent substrate is delivered to the mainanalysis channel from pump-module 406, which includes reservoir/ports406 a and 406 b, which provide the same function as ports 216 and 218 inFIG. 2 or ports 218 and 314 in FIG. 3. Specifically, a voltage gradientis applied along the length of the channel portion connecting these twoports, such that a positive pressure based flow is created in channel408. Prior to entering the analysis channel, the substrate is typicallydiluted with an appropriate assay buffer from pump module 410.Appropriate dilutions are obtained by modulating the amount of pressureproduced by each of pump modules 406 and 410.

Inhibitor is continuously transported into the analysis channel frompump module 412, and mixed with more diluent/assay buffer from pumpmodule 414. The dilute inhibitor is then contacted with the dilutesubstrate mixture in the analysis channel. At a downstream portion ofthe analysis channel, e.g., closer to waste reservoir 422, enzyme iscontinuously introduced into the analysis channel from pump module 416.Again, the enzyme may be delivered in full strength form or diluted withappropriate diluent/assay buffer from pump module 418. The relativerates at which the various materials are introduced into the analysischannel are controlled by the amount of pressure produced by each pumpmodule, which in turn is related to the amount of current applied acrossa given pump module. The results of the various inhibitor screens arethen determined at a detection point 420 along the analysis channel 404,e.g., using a fluorescence detection system.

This example merely illustrates one application of an integratedmicropump according to the present invention. It will be readilyappreciated upon reading the instant disclosure, that these micropumpshave a wide range of applications.

Although the present invention has been described in some detail by wayof illustration and example for purposes of clarity and understanding,it will be apparent that certain changes and modifications may bepracticed within the scope of the appended claims. All publications,patents and patent applications referenced herein are herebyincorporated by reference in their entirety for all purposes as if eachsuch publication, patent or patent application had been individuallyindicated to be incorporated by reference.

What is claimed is:
 1. A microfluidic device, comprising: a bodystructure comprising a first fluid containing microchannel having afirst segment and a second segment and a second fluid containingmicrochannel in fluid communication with said first microchannel at afirst location intermediate between the first and second segmentswherein said first segment comprises a substantially positive surfacecharge and said second segment comprises a substantially negativesurface charge; a means for applying a voltage gradient across saidfirst fluid containing microchannel and thereby creating electroosmoticflow in said first and second channel segments towards the firstlocation wherein said flow in the first and second channel segmentscauses a pressure based flow in the second microchannel.
 2. Themicrofluidic device of claim 1, wherein the means for applying a voltagegradient comprises a first and second electrode in electricalcommunication with first and second ends of the first fluid containingmicrochannel; and a power source electrically coupled to each of thefirst and second electrodes, whereby the power source is capable ofdelivering a different potential to each of the first and secondelectrodes.
 3. The microfluidic device of claim 1 wherein the surfacecharge of at least one of said first and second segments is modified bya surface coating.
 4. The microfluidic device of claim 1, wherein thesecond microchannel comprises a narrower cross-section than the firstmicrochannel.
 5. A method of inducing pressure flow in a microchannel,comprising: providing a first fluid-filled channel that is fluidlycoupled to a second fluid-filled channel at a first intersection, saidchannels comprising a fluid at a neutral pH; inducing electroosmoticflow across the first channel but not across the second channelwhereupon electroosmotic flow in the first channel translates topressure flow in the second channel.
 6. The method of claim 5, whereinthe step of inducing electroosmotic flow in the first channel and notthe second channel comprises applying an electric field along a lengthof the first channel but not the second channel.
 7. The method of claim6, wherein the applying of an electric field comprises providing firstand second electrodes in electrical contact with first and second pointsalong the first channel and applying a voltage potential to theelectrodes to apply an electric field along a length of the firstchannel.